The long term goal of this research is to understand the mechanism by which enteroviruses such as poliovirus (PV) and Coxsackievirus (CVB3) inactivate translation of nearly all cellular mRNA while stimulating efficient translation of virus RNA in infected HeLa cells, an event known as host translation shutoff. Previous work has shown that PV and CVB3 infection result in cleavage of the translation initiation factor eIF4G in reactions mediated by the virus 2Aprotease and other cellular proteases. This serves to inactivate de novo cap-dependent translation initiation however, existing models for cap-dependent translation initiation mechanism cannot account for continued translation of cellular mRNA in virus infections containing guanidine or methyl-quercitin in which eIF4G is totally cleaved. This translation is likely mediated by ribosome re-initiation via 5'-3' interactions on mRNA. Thus eIF4G cleavage plus additional events must take place to completely block host translation; we hypothesize that viral protease-mediated cleavage of poly(A)-binding protein (PABP) may be equally important. We have recently discovered that viral 3Cprotease can effectively block translation in reactions in which PABP is partly cleaved, yet eIF4GI and eIF4GII remain intact, thus providing direct evidence for the biological relevance of PABP cleavage. The region of PABP cleaved by viral protease is poorly understood but may function in the 60S ribosome joining step of translation and may regulate re-initiation of ribosomes on the same transcript in a 5'-3' interaction-dependent mechanism. Our discovery of PABP cleavage by viral proteases focuses attention on new areas of translation regulation involving 5'-3' interactions, an area of intense current interest. The aims in this proposal will (1) determine which pools of PABP are cleaved, (2) determine the impact of PABP cleavage on its various functions, particularly functions of PABP which may support ribosome re-initiation (3) determine the relative roles of PABP and eIF4G cleavage in translation control in vivo and (4) will also determine if PABP cleavage plays a role in the switch from viral translation to viral RNA replication.